1. Field of the Invention PA1 Q.sub.in : Amount of gas introduced from the inlet pipe PA1 P.sub.V : Saturated vapor pressure of raw material gas PA1 P.sub.B : Pressure inside the vessel
The present invention relates to an apparatus for vaporizing a liquid raw material, and more particularly to the apparatus applicable preferably for depositing a thin film by chemical vapor deposition.
2. Related Background Art
In a step of forming a thin film in the process for producing an integrated circuit with semiconductors, it is necessary to form a desired thin film on an etched wafer surface of uneven surface level while attaining a good step coverage. In a process for producing an integrated circuit of submicron level, whose minimum processing dimension is not more than 1 .mu.m, or of further deep submicron level, it is necessary a process which deposits a thin film selectively in contact holes or through holes. It is a step of forming a wiring metal film that requires the keenest selective growth in the process for producing an integrated circuit.
A thin film has been so far formed conventionally by sputtering. Sputtering is to deposit atoms sputtered from a target substance onto a substrate. Thus, not only is its step coverage poor, but it is also impossible to attain selective growth. On the other hand, chemical vapor deposition, which vaporizes a raw material, transports the vaporized raw material into a reaction space and deposits a desired thin film through chemical reactions of the raw material molecules on the substrate surface, provides distinguished step coverage and can provide selective growth. For example, an Al thin film growth process disclosed in Applied Physics Letters, Vol. 57, No. 12 (1990), Page 1221, can deposit monocrystalline Al selectively in very fine through holes and also can change the growth mode from a selected mode to a non-selected mode to completely flatten through holes.
In the chemical vapor deposition process (CVD), it is necessary to vaporize a raw material and transport the vaporized raw material into a reaction space. If the raw material exists in a gaseous state in the CVD, its transportation into the reaction space can be carried out with ease. For example, SiH.sub.4 is a gas at room temperature and thus can be filled in a high pressure gas cylinder. Therefore, it be transported into the reaction space at a desired rate by providing the high pressure gas cylinder with a pressure control valve and a flow meter. However, when a raw material is a liquid at room temperature, its transportation procedure is different from that for SiH.sub.4. For example, dimethylaluminun, which is a liquid at room temperature, is used as a raw material in the Al thin film growth. Such procedure is disclosed in detail in U.S. patent application Ser. No. 578,672, now U.S. Pat. No. 5,179,042 titled "Process for forming deposited film by use of alkyl aluminium hydride", filed Sep. 7, 1990 by the present inventors; U.S. patent application Ser. No. 587,045, now U.S. Pat. No. 5,180,687 titled "Deposited film formation method utilizing selective deposition by use of alkyl aluminum hydride", filed Sep. 24, 1990 by the present inventors, and U.S. patent application Ser. No. 586,877 titled "Gas feeding device and deposition film forming apparatus employing the same" filed Sep. 24, 1990, now Ser. No. 08/041,340filed Apr. 1, 1993; by the present inventors.
FIG. 3 shows a procedure for transporting a liquid raw material which is widely utilized, where a liquid raw material 2 is stored in a vessel 1 provided with an inlet pipe 103 and an outlet pipe 104, and the inlet pipe 103 is dipped into the liquid raw material 2. When a flow rate-controlled gas 105 is introduced from the inlet pipe 103, the gas 106 is discharged from the outlet pipe 104 through the liquid raw material 2. Since the gas 105 is passed through the liquid raw material 2, the gas 106 can be saturated with the vapor of the liquid raw material 2 and led to a reaction chamber of an apparatus for chemical vapor deposition. The amount of the transported raw material according to the foregoing procedure can be represented approximately by the following equation: ##EQU1## wherein Q: Amount of transported raw material
When the liquid raw material has a low saturation vapor pressure, the amount of transported raw material is small, as is seen from the equation (1), and in the worst case, the rate of deposition is governed by the amount of the transported raw material. When the pressure P.sub.B inside the vessel is constant, it is possible to introduce dilution gas into the reaction chamber to lower a proportion of the raw material partial pressure in the reaction chamber, but it is impossible to elevate the proportion. In order to enhance the transportation efficiency of the raw material, it is possible to elevate the temperature of the vessel to increase the saturated vapor pressure of the raw material, but the entire vessel and pipes must be heated. When the inlet gas flow rate is increased in the procedure as shown in FIG. 3, the vaporization rate of the liquid raw material cannot meet the inlet gas flow rate, and thus a sufficient amount of the raw material cannot be transported.
In the procedure as shown in FIG. 3, the amount of the gas 105 introduced into the inlet pipe 103 can be exactly determined, but the amount of the raw material discharged from the outlet pipe 106 can be only calculated from the equation (1), and its actual amount cannot be detected, that is, the exact amount of transported raw material can not be determined.
In the above-mentioned Al CVD using dimethylaluminum hydride (DMAH), deposition of high quality Al film can be conducted, but the saturated vapor pressure of DMAH at room temperature is as low as 2 Torr and there thus remains the problem that a large amount of DMAH can not be efficiently transported and no higher deposition rate can be attained.
As mentioned above, it is necessary in chemical vapor deposition using a liquid raw material in the technical field of semiconductor integrated circuits of higher speed and higher degree of integration due to the recent trends for miniaturization to transport a large amount of raw material stably and exactly. Accordingly, there still remain unsolved problems for the mass production.